The present invention relates to a balanced transmitting apparatus.
FIG. 18 is a view showing a current model on a transmission line. The transmission line is constituted by transmission lines (L1, L2) comprising a pair of conductors balanced relative to the earth ground.
In FIG. 18, a pair of voltage sources (e10, e20) are provided at left ends of the transmission lines. Characteristics of waveform, amplitude, phase and the like of e10 and e20 are equal and voltages generated from the respective voltage sources (e10 , e20) (respective voltages are designated by notation “e”) are applied to the respective lines (L1, L2).
A load impedance (regular load) ZL is provided at right ends of the transmission lines. Differential (normal) mode currents (+Id, −Id) directed inverse to each other flow in the transmission lines (L1, L2).
Incidentally, parasitic impedances (impedances to the ground) Z1 through Z3 are interposed between the transmission lines and the ground (earth).
In this case, when impedance values (also the values are designated as Z2, Z3 ) of the parasitic impedances Z2, Z3 are not equal, a common mode current Ic which is a circulating current by way of the parasitic impedances (impedances to the ground) Z1 through Z3 as well as the ground flows.
In the case of transmission lines having a sufficiently high balance, it can be regarded that Z2=Z3 and the common mode current Ic does not flow. However, in the case of lines having a comparatively low balance, Z2≠Z3, and the common mode current Ic flows.
A current amount of the common mode current Ic can be represented as follows. In Equation (1) shown below, notation e designates respective voltages generated from the respective voltages sources (e10, e20).
                    Ic        =                              e            ·                          (                                                Z                  ⁢                                                                          ⁢                  2                                -                                  Z                  ⁢                                                                          ⁢                  3                                            )                                                          Z              ⁢                                                          ⁢                              1                ·                Z                            ⁢                                                          ⁢              2                        +                          Z              ⁢                                                          ⁢                              2                ·                Z                            ⁢                                                          ⁢              3                        +                          Z              ⁢                                                          ⁢                              3                ·                Z                            ⁢                                                          ⁢              1                                                          (        1        )            
The common mode current Ic is bifurcatedly branched to flow equally at respectives of the pair of transmission lines (L1, L2). Therefore, a common mode current (the current amount becomes Ic/2) flows in the respective transmission lines (L1, L2) in the same direction.
Further, the common mode current becomes a factor of producing radiation from the line. That is, when signals having the same amplitude and inverse phases are superposed on respectives of the pair of lines, although radiation components radiated from the respective lines are canceled by each other when a balanced state is maintained, in an unbalance state, there is a case in which the radiation components are not canceled by each other completely but the radiation components leak to outside.
FIG. 19 is a view showing a constitution of an example of a balanced transmission circuit of a background art. According to the balanced transmission circuit of FIG. 19, an unbalance transmission signal (transmission signal of one route) is converted into a balanced signal by way of a transformer (800) and balanced transmission is carried out by way of the pair of transmission lines (PL1, PL2).
The common mode current can be suppressed to be low by promoting the degree of balance by using the transformer having a high degree of balance or inserting a common mode choke coil operating to remove the common mode current.
Further, when lines having a high balance (for example, lines exclusive for communication of twist pair cables or the like, for example, Ethernet (registered trade mark)) are used as the transmission lines (PL1, PL2), the common mode current generated on the lines are suppressed to a sufficiently low level and therefore, the problem of radiation is not posed.
Further, with regard to radiation caused by the unbalance component of the transmission lines, a detailed description is given in Nonpatent Reference 1.
[Nonpatent Reference 1] “Electromagnetic noise problem of information communication system and technology dealing therewith” by Ideguchi, K. et al, Morikita shuppan, Nov. 25, 1997, P. 99-134
In the case of lines having a high balance (for example, lines exclusive for communication), the common mode current on the lines are sufficiently suppressed from being generated and therefore, the problem of radiation is not posed. Therefore, it becomes important to reduce the common mode current as less as possible by promoting a degree of balance of the transmission per se as much as possible by devising a constitution of an unbalance-balance converting circuit.
However, when lines having a comparatively low degree of balance are used as communication lines, even when the degree of balance of the transmission signal is promoted, generation of the common mode current cannot necessarily be reduced.
That is, since the communication lines per se are provided with an unbalance characteristic, there is a case in which an effect of sufficiently suppressing the common mode current cannot be achieved by the technology of the background art of “promoting a degree of balance of the transmission signal per se”.